I. Field of the Invention:
This invention relates generally to temperature control apparatus and more specifically to an open loop control circuit for maintaining the temperature of a bonding blade at a desired preset value.
II. Discussion of the Prior Art:
In my earlier patent, U.S. Pat. No. 4,189,340, there is described a thermoplastic bonding machine especially designed to bond or weld together two three-dimensional articles formed of foamed or expanded polystyrene plastic. The two articles to be joined are held in a fixture with the edges of the articles to be bonded aligned. Thereafter, a heated blade is passed through the juncture zone at a predetermined rate so as to contact both of the abutting surfaces of the two articles to be bonded. The heated knife first fuses the surfaces of the articles to be joined and subsequently a programmed motion is applied to the articles to force them together while the fused portions solidify. When working with foamed plastic material such as foamed polystyrene, it is known that accurate control must be maintained over the temperature and rate of travel to the thermal bonding blade as it traverses the juncture zone to ensure acceptable bonding of the two articles together. The present invention addresses the former aspect, i.e., the temperature control of the bonding blade. However, while the preferred embodiment will be explained in connection with a thermoplastic bonding application, it is conceivable that the present invention may find wider application such as in the cutting of thermoplastic materials or the like, that no limitation to the use in bonding apparatus is intended.
In heat sealing or thermal bonding systems it is essential that proper attention be paid to the blade temperature and the speed at which the blade traverses the material being operated upon. If the blade temperature is too high for a given speed, too much of the material becomes fused at the juncture zone and a density discontinuity in the resulting article results. However, if the temperature is too low, insufficient fusion takes place and an imperfect joint or weld results.
In the Gassner U.S. Pat. No. 3,016,085 there is described an apparatus for heat sealing a thermoplastic web in which the heat is controlled as a function of the speed of the web past the heating element. At such, a relatively expensive control circuit involving a tachometer-type dynamo and a magnetic amplifier circuit having means for establishing a set-point is required.
In the case of the present invention, the workpieces to be joined may be moved relative to the heating blade at a constant speed and means are provided for controlling the temperature of the blade by monitoring the change in length of that blade occasioned by changes in its temperature and then utilizing the resulting change in length to vary the amount of current flowing through the blade. Alternatively, the length changes of the thermal bonding blade occasioned by blade temperature changes can be monitored and those length changes may, in turn, be used to control the speed of movement of the workpieces relative to the blade. Hence, precise control over bonding temperatures for fixed or variable travel speeds may be obtained without the need for costly workpiece speed sensing apparatus required in prior art systems.